The invention relates to a method of exchanging a printing cylinder sleeve in a printing machine which has a central cylinder and a plurality of ink units that are arranged to be adjusted to the central cylinder and each comprise a plate cylinder having a printing cylinder sleeve mounted on a mandrel, in which method the plate cylinder is moved away from the central cylinder and the printing cylinder sleeve is axially withdrawn from the mandrel after a bearing has been detached.
U.S. Pat. No. 5,832,829 (EP-A-0 812 681) discloses a flexographic printing machine having a plurality of ink units arranged at the periphery of a central cylinder, wherein the plate cylinder of each ink unit has a printing cylinder sleeve that is exchangeably mounted on a mandrel which is fixed in the machine frame. When the printing cylinder sleeve has to be exchanged, in order to, for example, prepare the machine for another print job, the plate cylinder is moved away from the periphery of the central cylinder, and, on the operating side of the printing machine, a bearing for the plate cylinder is detached and removed, for example by being shifted aside, so that the printing cylinder sleeve can axially be withdrawn from the mandrel. The mandrel is supported in a cantilever fashion on the opposing side of the printing machine and remains in the machine frame throughout the exchange process.
The printing cylinder sleeve may be a printing sleeve, on which the printing plates or blocks are mounted, or an intermediate sleeve which itself carries on its outer periphery a sleeve with the blocks mounted thereon.
In the paper xe2x80x9cFlexo+Tief-Druckxe2x80x9d, 3-2000, May 2000, pages 148 and 149, a printing machine of the assignee has been described, wherein printing sleeves and intermediate sleeves with varying diameters may be clamped on the mandrel. The intermediate sleeve can be clamped on the mandrel by means of an hydraulic system. The sleeves are either thrust onto the intermediate sleeve or directly onto the mandrel, depending on their diameter. For facilitating the exchange of the sleeve, a compressed air system is provided, whereby compressed air is radially blown out of the mandrel and out of the intermediate sleeve, as the case may be, so that the printing sleeve may be slid on air cushion to be withdrawn from the mandrel or the intermediate sleeve by hand with low frictional resistance.
By utilizing a printing cylinder sleeve made of a carbon fiber compound material, a weight reduction can be achieved, which facilitates the handling of the printing cylinder sleeve and at the same time reduces the load on the mandrel that is cantilevered in the machine frame.
Assignee""s EP-A-1 090 754 describes an apparatus which facilitates the manual exchange of the printing cylinder sleeve by providing, on the side opposite to the operating side of the machine, an ejector by which the printing cylinder sleeve to be exchanged can be ejected to some extent towards the operating side, so that it can more readily be gripped and withdrawn completely. When a new printing cylinder sleeve has been thrust onto the mandrel, it can be gripped by a claw of the ejector and can be drawn into the final position.
In spite of these measures for facilitating the handling, the exchange of the printing cylinder sleeves, when the machine is prepared for another print job, is still a relatively time-consuming operation which causes long production stops of the printing machine.
It is an object of the invention to provide a method by which, at least in some cases, the time of the production stop, when the printing machine is prepared for another job, can be shortened.
According to the invention, this object is achieved by the feature that the exchange of the printing cylinder sleeve is performed while the printing machine is running, and the plate cylinder is moved away from the central cylinder to such an extent that a space, which permits to safely grip the printing cylinder sleeve, is formed between the printing cylinder sleeve and the rotating central cylinder.
This method permits to shorten the time of a production stop in all those cases in which not all print units of the machine have been used for the previous job.
In this case, it is possible to exchange the printing cylinder sleeves of the non-used ink units before the running job has come to an end. Thus, the printing machine can be converted completely or partly for the print job to be executed next, before the running job is completed, so that the times of standstill can be reduced considerably. If, for example, in a printing machine having six ink units, two print jobs have to be executed one after the other, and only three colors are used in each job, then the three ink units which are not needed for the first job can already be converted for the next job, so that no time is needed any longer for exchanging the printing cylinder sleeves between the end of the first job and the beginning of the second job.
In most conventional printing machines, the plate cylinder is disposed at only a short distance from the peripheral surface of the central cylinder, even when the plate cylinder has been moved away from the central cylinder. Since the central cylinder rotates at high speed, there is a risk of damages or injuries when the printing cylinder sleeve is exchanged while the machine is running and when the printing cylinder sleeve or the equipment of handling the same or the hand of the operator, who exchanges the printing cylinder sleeves, touches the rotating central cylinder. This is why, according to the invention, the plate cylinder is moved so far away from the central cylinder that a sufficient safety space is formed between the printing cylinder sleeve and the periphery of the central cylinder. If no additional safety measures are provided, this space should be at least 120 mm. This implies a construction of the printing machine which permits to displace the plate cylinder far enough from the central cylinder.
The invention further provides a printing machine in which a safeguard is provided at the periphery of the central cylinder at the end thereof facing towards the operating side of the machine. This permits to safely exchange the printing cylinder sleeve, while the printing machine is running, even in cases in which, for constructional reasons, a safety distance of 120 mm or more between the plate cylinder and the central cylinder cannot be provided.
In a specific embodiment, the safeguard is arranged movably, and it is brought into the operative position only when the plate cylinder has been moved away from the central cylinder. In this case, the safeguard may have such a construction that, in its operative position, it projects into the space between the plate cylinder and the central cylinder and thereby shields the peripheral surface of the rotating central cylinder partly or completely.